There's a place to choose a path in the Audio Spectrum effect menu (1) and I have created such a path (2), however when I try to choose it in (1) only "None" shows up. The Mask 1 is in a Null Object Layer. What am I doing wrong?

you have set the spectrum effect to a layer named "white" (which is layer number 3 in the stacking order), but the mask you are showing belongs to another layer named "spetru-something" (layer number 9 in the stacking order). the path needs to be in the same layer as the spectrum effect. of course don't set an effect to null layer because you won't see it since null layer's transparency is always at 0%